1. Field of the Invention
The invention relates to apparatus for a drive circuit that provides bursts, rather than continuously alternating amounts, of energy for use in driving a flow tube (conduit) in a Coriolis meter and to methods for use in such a circuit.
2. Description of the Prior Art
Currently, Coriolis meters are finding increasing use as an accurate way to measure the mass flow rate and/or density of various process fluids in many applications.
Generally speaking, a Coriolis mass flow rate meter, such as that described in U.S. Pat. No. 4,491,025 (issued to J. E. Smith et al on Jan. 1, 1985), contains one or two parallel conduits, each typically being a U-shaped flow conduit or tube. Each flow conduit is driven to oscillate about an axis to create a rotational frame of reference. For a U-shaped flow conduit, this axis can be termed the bending axis. As process fluid flows through each oscillating flow conduit, movement of the fluid produces reactionary Coriolis forces that are orthogonal to both the velocity of the fluid and the angular velocity of the conduit. These reactionary Coriolis forces, though quite small when compared to the force at which the conduits are driven, nevertheless cause each conduit to twist about a torsional axis that, for a U-shaped flow conduit, is normal to its bending axis. The amount of twist imparted to each conduit is related to the mass flow rate of the process fluid flowing therethrough. This twist is frequently measured using velocity signals obtained from magnetic velocity sensors that are mounted to one or both of the flow conduits in order to provide a complete velocity profile of the movement of each flow conduit with respect to either the other conduit or a fixed reference. In dual tube meters, both flow conduits are oppositely driven such that each conduit oscillates (vibrates) as a separate tine of a tuning fork. This "tuning fork" operation advantageously cancels substantially all undesirable vibrations that might otherwise mask the Coriolis force.
In such a Coriolis meter, the mass flow rate of a fluid that moves through the meter is proportional to the time interval that elapses between the instant one point situated on a side leg of a flow conduit crosses a pre-determined location, e.g. a respective mid-plane of oscillation, until the instant a corresponding point situated on the opposite side leg of the same flow conduit, crosses its corresponding location, e.g. its respective mid-plane of oscillation. For parallel dual conduit Coriolis mass flow rate meters, this interval is equal to the phase difference between the velocity signals generated for both flow conduits at the fundamental (resonant) frequency at which these flow conduits are driven. In addition, the resonant frequency at which each flow conduit oscillates depends upon the total mass of that conduit, i.e. the mass of the conduit itself, when empty, plus the mass of any fluid flowing therethrough. Inasmuch as the total mass varies as the density of the fluid flowing through the tube varies, the resonant frequency likewise varies with any changes in fluid density and as such can be used to track changes in fluid density.
As noted above, these mass flow and density relationships inherent in a Coriolis meter require that each flow conduit in the meter must be driven to resonantly vibrate in order for the meter to properly operate. To ensure that proper vibratory motion is initiated in, for example a dual tube Coriolis meter, and thereafter maintained during operation of the meter, the meter contains an appropriate drive mechanism that is mounted to both of the flow conduits typically between corresponding extremities of these conduits. The drive mechanism frequently contains any one of many well known arrangements, such as a magnet mounted to one conduit and a coil mounted to the other conduit in an opposing relationship to the magnet. A drive circuit continuously applies a periodic, typically sinusoidally or square shaped, drive voltage to the drive mechanism. Through interaction of the continuous alternating magnetic field produced by the coil in response to the periodic drive signal and the constant magnetic field produced by the magnet, both flow conduits are initially forced to vibrate in an opposing sinusoidal pattern which is thereafter maintained. Inasmuch as the drive circuit tightly synchronizes the frequency of the drive signal to essentially match the resonant frequency of the conduits, both flow conduits are kept in a state of opposing substantially resonant sinusoidal motion.
One known drive circuit currently in use today and typified by that disclosed in, for example, U.S. Pat. No. 4,777,833 (issued to B. L. Carpenter on Oct. 18, 1988--hereinafter referred to as the '833 Carpenter patent--and currently owned by the present assignee) utilizes an analog drive circuit. Specifically, this circuit utilizes a synchronous analog amplifier to generate a continuous square wave with two analog levels that each equally change based upon a simultaneously occurring difference between an analog reference voltage and a flow conduit position signal. As the magnitude of this difference increases (decreases), based upon decreasing (increasing) amplitudes of the oscillatory movement of the flow conduits which results from, for example, increases (decreases) in the density in the process fluid that simultaneously flows through the flow conduits, positive and negative drive levels produced by the synchronous amplifier corresponding and equally increase (decrease) to once again restore the amplitude of the oscillatory flow tube movement to its proper level. Various analog components, such as inter alia amplifiers, buffers, a phase shifter and an edge detector, are used to appropriately determine this difference based upon the analog reference level and one of the velocity sensor signals, typically a left velocity sensor signal, produced within the meter.
Unfortunately, analog drive circuits used in Coriolis meters and typified by that described in the '833 Carpenter patent suffer from various drawbacks.
First, analog drive circuits, particularly those which provide an alternating square shaped drive signal to the coil, do not permit the energy that is applied to the drive coil to be precisely controlled by the drive circuit itself at any one instant during the signal. With these circuits, the drive signal is merely set to alternate between two levels that are static within any one drive cycle. Precise control over the energy supplied to drive coil by the drive circuit itself has proven to be particularly important in those applications, such as intended use of the meter particularly the mechanical Coriolis metering assembly itself in a hazardous environment, where a critical need exists to always limit this energy to as low a value as is realistically possible. While intrinsic safety barriers are used in these applications to limit the energy that would flow to the drive coil located in a hazardous area to below a pre-defined maximum amount and in doing so perform extremely well, it would be preferable to further limit the energy at its source, if possible, i.e. drive circuit, and rely on the barrier as a back-up protective device rather than as a main mechanism for limiting the energy.
Second, analog drive circuits generally tend to be complex and require a multitude of parts which adds to the manufacturing cost of the meter electronics.
Third, discrete analog components, such as those used in a drive circuit, may exhibit undesirable temperature, aging and/or drift characteristics any one of which might, over time, cause the output produced by such a component to vary. These affects can be minimized to a certain and usually acceptable extent by using components with matched temperature characteristics, appropriate temperature compensation circuits and/or sufficiently frequent re-calibration. However, use of matched components further increases the cost of the meter electronics, while temperature compensation circuits often require additional components which increase the parts count as well as the manufacturing cost of the drive circuit. Re-calibration disadvantageously increases the costs associated with actual use of the meter.
Therefore, a need exists in the art for a simple and inexpensive flow tube drive circuit particularly suited for use in a Coriolis meter that provides substantially accurate control over the amount of energy that is to be applied to the drive coil at any instant, has a reduced parts count and cost over analog drive circuits known in the art, and does not appreciably, suffer, if at all, from temperature, aging and/or drift affects which are commonly associated with analog drive circuits known in the art.